How do I create a constant in Python?

Is there a way to declare a constant in Python? In Java we can create constant values in this manner:

public static final String CONST_NAME = "Name";

What is the equivalent of the above Java constant declaration in Python?

---

No there is not. You cannot declare a variable or value as constant in Python. Just don't change it.

If you are in a class, the equivalent would be:

class Foo(object):
    CONST_NAME = "Name"

if not, it is just

CONST_NAME = "Name"

But you might want to have a look at the code snippet Constants in Python by Alex Martelli.

---

There's no const keyword as in other languages, however it is possible to create a Property that has a "getter function" to read the data, but no "setter function" to re-write the data. This essentially protects the identifier from being changed.

Here is an alternative implementation using class property:

Note that the code is far from easy for a reader wondering about constants. See explanation below

def constant(f):
    def fset(self, value):
        raise TypeError
    def fget(self):
        return f()
    return property(fget, fset)

class _Const(object):
    @constant
    def FOO():
        return 0xBAADFACE
    @constant
    def BAR():
        return 0xDEADBEEF

CONST = _Const()

print CONST.FOO
##3131964110

CONST.FOO = 0
##Traceback (most recent call last):
##    ...
##    CONST.FOO = 0
##TypeError: None

Code Explanation:

Define a function constant that takes an expression, and uses it to construct a "getter" - a function that solely returns the value of the expression.

The setter function raises a TypeError so it's read-only

Use the constant function we just created as a decoration to quickly define read-only properties.

---

And in some other more old-fashioned way:

(The code is quite tricky, more explanations below)

class _Const(object):
    @apply
    def FOO():
        def fset(self, value):
            raise TypeError
        def fget(self):
            return 0xBAADFACE
        return property(**locals())

CONST = _Const()

print CONST.FOO
##3131964110

CONST.FOO = 0
##Traceback (most recent call last):
##    ...
##    CONST.FOO = 0
##TypeError: None

Note that the @apply decorator seems to be deprecated.

To define the identifier FOO, firs define two functions (fset, fget - the names are at my choice).

Then use the built-in property function to construct an object that can be "set" or "get".

Note hat the property function's first two parameters are named fset and fget .

Use the fact that we chose these very names for our own getter & setter and create a keyword-dictionary using the ** (double asterisk) applied to all the local definitions of that scope to pass parameters to the property function

---

In Python instead of language enforcing something, people use naming conventions eg __method for private methods and using _method for protected methods.

So in same manner you can simply declare the constant as all caps eg

MY_CONSTANT = "one"

If you want that this constant never changes, you can hook into attribute access and do tricks, but a simpler approach is to declare a function

def MY_CONSTANT():
    return "one"

Only problem is everywhere you will have to do MY_CONSTANT(), but again MY_CONSTANT = "one" is the correct way in python(usually).

You can also use namedtuple to create constants:

>>> from collections import namedtuple
>>> Constants = namedtuple('Constants', ['pi', 'e'])
>>> constants = Constants(3.14, 2.718)
>>> constants.pi
3.14
>>> constants.pi = 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

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